Typical turbine rotor blades in a gas turbine engine are hollow and include one or more cooling circuits therein through which is circulated cooling air bled from a compressor of the engine. The art of cooling turbine blades is very crowded and includes various holes and turbulators for cooling the various portions of the airfoil.
The airfoil portion of a typical blade has pressure and suction sides joined together at leading and trailing edges and extend from a root to tip of the airfoil. During operation, hot combustion gases flow over the airfoil from the combustor of the engine, with the airfoil extracting energy therefrom for powering a rotor disk to which it is attached. The surface of the airfoil experiences varying heat input at different locations thereof, and therefore various cooling arrangements are tailored to the airfoil for suitably cooling the various portions thereof.
In a typical configuration, the airfoil includes a cooling air passage extending radially along its trailing edge, with the trailing edge having a plurality of radially spaced apart axial cooling holes which cool the airfoil trailing edge. The airfoil tip also may include radially extending cooling holes therethrough for cooling thereof. However, the intersection between the airfoil trailing edge and the airfoil tips defines a tip corner which also typically requires cooling thereof. In one conventional configuration, cooling holes radiate outwardly through the tip corner in a fan configuration from the axial, trailing edge cooling holes to the radial, tip cooling holes for effectively cooling the tip corner.
The various cooling holes in the airfoil are formed or drilled using conventional processes such as laser drilling and electrical discharge machining (EDM) which are effective for forming the relatively small diameters required. However, these processes limit the length or depth of the cooling holes which may be formed therewith. Since the fan shaped cooling holes through the airfoil tip Corner are significantly longer than the adjacent cooling holes in the trailing edge and airfoil tip, they require a different process for forming their longer lengths.
For example, conventional electrostream (ES) drilling may be used for forming the fan shaped cooling holes through the airfoil tip corner. Electrostream drilling uses a suitable electrolytic liquid and electricity to drill the cooling holes using an electrochemical reaction in a conventionally known process. In that process, glass nozzles are used for directing the electrolyte flow against the airfoil for drilling the required cooling holes therethrough. However, in order to form the fan shaped cooling holes in the airfoil tip corner, the electrostream glass nozzles must be inclined relative to the outer surface of the tip corner at angles substantially less than 90.degree. or normal thereto which results in substantial breakage of the glass nozzles during the drilling process, with an attendant increase in manufacturing costs, which is undesirable.